Axial pump

ABSTRACT

A rotary blood pump includes a pump housing for receiving a flow straightener, a rotor mounted on rotor bearings and having an inducer portion and an impeller portion, and a diffuser. The entrance angle, outlet angle, axial and radial clearances of blades associated with the flow straightener, inducer portion, impeller portion and diffuser are optimized to minimize hemolysis while maintaining pump efficiency. The rotor bearing includes a bearing chamber that is filled with cross-linked blood or other bio-compatible material. A back emf integrated circuit regulates rotor operation and a microcomputer may be used to control one or more back emf integrated circuits. A plurality of magnets are disposed in each of a plurality of impeller blades with a small air gap. A stator may be axially adjusted on the pump housing to absorb bearing load and maximize pump efficiency.

This application is a continuation of U.S. application Ser. No.08/153,595 filed Nov. 10, 1993 for a "ROTARY BLOOD PUMP", now U.S. Pat.No. 5,527,159.

FIELD OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435; 42 U.S.C. 2457).

TECHNICAL FIELD

The present invention relates generally to rotary blood pumps. Morespecifically, the present invention relates to an axial flow ventricleassist blood pump.

BACKGROUND OF THE INVENTION

Ventricle assist devices are frequently used to boost blood circulationto assist a heart which still functions but is not pumping sufficientblood for adequate circulation. Rotary pumps are often the preferredtype of pump for use as a ventricle assist device compared to othertypes of pumps which may use pistons, rollers, diaphragms, or compliancechambers. This is partially because rotary pumps may be manufactured ata relatively low cost and are typically less complex than other types ofpumps. Other types of blood pumps may cost up to $50,000 per unit and,due to financial limitations, are therefore not available for use uponthe large population which could benefit from such pumps. Rotary bloodpumps are increasingly used not only for ventricular assistapplications, but also for cardiopulmonary bypass procedures andpercutaneous cardiopulmonary support applications in emergency cases.

Clinical uses of rotary pumps are conventionally limited to a few daysdue to shortcomings of these pumps. A non-comprehensive list of suchproblems or shortcomings would include the following: (1) blood damagewhich may occur when blood comes into contact with rotor bearings, (2)the need for bearing purge systems which may require percutaneous(through the skin) saline solution pump systems, (3) bearing seizureresulting from the considerable thrust and torque loads, or from driedblood sticking on the bearing surfaces, (4) problems of blood damage(hemolysis) and blood clotting (thrombosis) caused by relativerotational movement of the components of the pump, (5) pump and controlsize and shape limitations necessary for implantation or convenientmobility, (6) weight limitations for implantation to avoid tearing ofimplant grafts due to inertia of sudden movement, (7) difficulty incoordinating and optimizing the many pump design parameters which mayaffect hemolysis, (8) high power consumption that requires a largerpower supply, (9) motor inefficiency caused by a large air gap betweenmotor windings and drive magnets, (10) heat flow from the device to thebody, (11) complex Hall Effect sensors/electronics for rotary control,(12) the substantial desire for minimizing percutaneous (through theskin) insertions, including support lines and tubes, and (13) large pumpand related hose internal volume which may cause an initial shock whenfilled with saline solution while starting the pump.

Although a significant amount of effort has been applied to solving theproblems associated with rotary pumps, there is still a great demand fora safe, reliable, and durable blood pump that may be used for longerterm applications. The estimated need for a simple and long termventricle assist device (VAD) is presently projected at between 50,000and 100,000 patients per year in the United States alone.

The following patents describe attempts made to solve problemsassociated with rotary blood pumps, including ventricle assist devices.

U.S. Pat. No. 4,625,712 to R. K. Wampler discloses a full-flow cardiacassist device for cariogenic shock patients which may be inserted intothe heart through the femoral artery and driven via a flexible cablefrom an external power source. A catheter attached to the pump suppliesthe pump bearings with a blood-compatible purge fluid to preventthrombus formation and the introduction of blood elements betweenrotating and stationary elements. Due to the very small diameter of thepump, rotational speeds on the order of 10,000 to 20,000 rpm are used toproduce a blood flow of about four liters per minute.

U.S. Pat. No. 4,957,504 to W. M. Chardack discloses an implantable bloodpump for providing either continuous or pulsatile blood flow to theheart. The pump includes a stator having a cylindrical opening, anannular array of electromagnets disposed in a circle about the statorconcentric with the cylindrical opening, a bearing carried by the statorand extending across the cylindrical opening, and a rotor supported bythe bearing. The rotor is in the form of an Archimedes screw and has apermanent magnet in its periphery which lies in the same plane as thecircular array of electromagnets for being driven in stepper motorfashion.

U.S. Pat. No. 4,944,722 to J. W. Carriker discloses a percutaneouslyinsertable intravascular axial flow blood pump with a rotor extensionand drive cable fitting designed so that the thrust bearing surfaces ofthe purge seal and cable fitting can be preloaded.

U.S. Pat. No. 4,817,586 to R. K. Wampler discloses an intravascular flowblood pump having blood exit apertures in the cylindrical outside wallof the pump housing between the rotor blades and the rotor journalbearing.

U.S. Pat. No. 4,908,012 to Moise et al. discloses an implantableventricular assist system having a high-speed axial flow blood pump. Thepump includes a blood tube in which the pump rotor and stator arecoaxially contained, and a motor stator surrounding the blood duct. Apermanent magnet motor rotor is integral with the pump rotor. Purgefluid for the hydrodynamic bearings of the device and power for themotor are preferably percutaneously introduced from extra-corporealsources worn by the patient.

U.S. Pat. No. 4,779,614 to J. C. Moise discloses an implantable axialflow blood pump which includes a magnetically suspended rotor of arelative small diameter disposed without bearings in a cylindrical bloodconduit. Neodymium-boron-iron rotor magnets allow a substantial gapbetween the static motor armature and the rotor. Magnetically permeablestrips in opposite ends of the pump stator blades transmit to Hallsensors the variations in an annular magnetic field surrounding therotor and adjacent the ends of the pump stator blades.

U.S. Pat. No. 5,049,134 to Golding et al. discloses a seal freecentrifugal impeller supported in a pump housing by fluid bearingsthrough which a blood flow passageway is provided.

U.S. Pat. No. 4,382,199 to M. S. Isacson discloses a hydrodynamicbearing system for use with a left ventricle assist device. The bearingsare formed by the fluid in the gap between the rotor and the stator.

U.S. Pat. No. 4,135,253 to Reich et al. discloses a centrifugal bloodpump provided with a magnetic drive system which permits a synchronousmagnetic coupling with a separate power unit disposed immediatelyadjacent the pump housing but outside of the skin surface. The pump hasa single moving part which includes the combination of an impellerconnected to a magnetic drive rotor. The magnetic drive system floats ona fluid surface of saline solution.

U.S. Pat. No. 4,507,048 to J. Belenger discloses a centrifugal bloodpump with a bell-shaped housing having a suction inlet at the apex and atangential outlet adjacent the base. A conical rotator is driven byspaced permanent magnets embedded in the base of the rotator and anexternally generated rotating magnetic field.

U.S. Pat. No. 4,688,998 to Olsen et al. discloses a pump with amagnetically suspended and magnetically rotated impeller. The impellermay be configured for axial flow with a hollow, cylindrical-typeimpeller with impeller vanes on the internal surface thereof. Theimpeller includes a plurality of internally embedded, permanent magnetsthat cooperate with electromagnets for drive and position control of theimpeller.

U.S. Pat. No. 4,763,032 to Bramm et al. discloses a magnetic rotorbearing for suspending a rotor for an axial or radial-centrifugal bloodpump in a contact-free manner, and comprising a permanent andelectromagnetic arrangement.

U.S. Pat. No. 4,846,152 to Wampler et al. discloses a miniaturehigh-speed intravascular blood pump with two rows of rotor blades and asingle row of stator blades within a tubular housing. The first row ofblades has no provision for a variable pitch, but produces a mixedcentrifugal and axial flow by increasing hub diameter. The second row ofblades is axially spaced from the first row, and produces a purely axialflow. The stator blades are reverse twisted to straighten and slow theblood flow.

U.S. Pat. No. 4,944,748 to Bramm et al. discloses an impeller in a bloodpump supported by permanent magnets on the impeller and pump housing,which are stabilized by an electromagnet on the pump housing. Theimpeller is rotated magnetically, and stator coils in the housing aresupplied with electric currents having a frequency and amplitudeadjusted in relation to blood pressure at the pump inlet.

U.S. Pat. No. 4,994,078 to R. K. Jarvik discloses an electricallypowered rotary hydrodynamic pump having motor windings and laminationsdisposed radially about an annular blood channel, and having a motorrotor disposed therein such that an annular blood channel passes throughthe gap between the motor rotor and the windings.

U.S. Pat. No. 5,055,005 to Kletschka discloses a fluid pump with anelectromagnetically driven rotary impeller levitated by localizedopposed fluid forces.

In spite of the effort evidenced by the above patents, there remains theneed for an improved rotary pump for use as a ventricle assist devicethat is reliable, compact, requires limited percutaneous insertions, andproduces fewer blood damage problems. Those skilled in the art willappreciate the features of the present invention which addresses theseand other problems.

STATEMENT OF THE INVENTION

The present invention provides a rotary blood pump which includes a pumphousing defining a blood flow path therethrough. A first stator having afirst stator field winding is used to produce a first stator magneticfield. A first rotor is mounted within the pump housing for rotation inresponse to the first stator magnetic field. The first rotor carries ablade thereon to propel blood through the pump housing along the bloodflow path. At least one magnet is secured to the first rotor andproduces a first rotor magnetic field that passes through the firststator field winding during rotation of the first rotor to therebyinduce a back emf within the first stator field winding. Back emf sensorcircuitry connected to the first stator field winding senses back emfproduced during the rotation of the first rotor within the pump housing.In one embodiment of the present invention, at least one magnet isimplanted in each of a plurality of impeller blades to producerotational movement of the rotor.

The rotary pump also includes, in a preferred embodiment, an inducerportion of the rotor having a plurality of inducer blades equidistantlydisposed about a circumference of the rotor with each inducer bladehaving a variable pitch along its axial length. An interconnection bladeportion connects at least one of the inducer blades to at least oneimpeller blade to form a continuous blade extending through said inducerand impeller portions of the rotor.

The rotary pump preferably includes a first rotor bearing for rotatablysupporting the rotor within the pump housing. The rotor bearing includesa curved bearing surface fixed with respect to the pump housing. Thefixed bearing surface receives a rotating member having a rotatingcurved bearing surface. The fixed bearing surface and the rotatingcurved bearing surface define a bearing chamber which is filled withbio-compatible material.

An object of the present invention is to provide an improved rotaryblood pump.

A further object is to provide an improved control circuit forcontrolling a rotor within the pump in response to a back emf producedin stator windings.

Another object is to provide an improved rotor bearing for whichcross-linked blood forms a bearing surface and blood seal.

Yet another object of the present invention is to provide a method foroptimizing pump parameters to reduce blood hemolysis.

A feature of the present invention is a reduced air gap betweenpermanent magnets on the rotor and the stator winding.

Another feature of the present invention is an impeller having avariable pitch blade.

Another feature of the present invention is a back emf integratedcircuit for controlling rotor operation.

An advantage of the present invention is improved rotor control.

Another advantage of the present invention is quantifiably reduceddamage to blood.

Yet another advantage of the present invention is an elimination of theneed for a bearing purge system requiring saline carrying tubespenetrating through the skin.

Other objects, features and intended advantages of the present inventionwill be readily apparent by the references to the following detaileddescription in connection with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view, partially in section, of a rotary bloodpump in accord with the present invention;

FIG. 2 is an end view of a stator showing stator laminations stacked toform a skewed stator;

FIG. 2A is an elevational view indicating the skewed path of statorfield windings through the stator;

FIG. 3 is an elevational view, partially in section, of an alternativeembodiment rotary blood pump having distinct impeller and inducerblades;

FIG. 4 is a cross-sectional view of a portion of an impeller showingnon-radiused blade tips;

FIG. 5 is a block diagram of a control system including a back emfintegrated circuit and a microprocessor;

FIG. 6A is an elevational view, partially in section, of a ball-socketrotor bearing having a bearing chamber filled with bio-compatiblematerial;

FIG. 6B is an elevational view, partially in section, of a shaft journalrotor bearing having a bearing chamber filled with bio-compatiblematerial;

FIG. 6C is an elevational view, partially in section, having a rotorbearing washed from increased blood flow caused by a bend in the pumphousing;

FIG. 6D is an elevational view, partially in section, showing a rotorbearing shaft having blood flow passages along the shaft periphery;

FIG. 6E is a cross-sectional view along line 6E--6E;

FIG. 6F is an elevational view, partially in section, of a rotor bearingfor supporting the rotor in cantilevered fashion;

FIG. 7 is a chart showing optimum pump parameter components determinedfrom methods of optimizing pump parameters to minimize hemolysis;

FIG. 8 shows the test matrix of the present invention for optimizingpump parameters to minimize hemolysis while maximizing pump efficiency;

FIG. 9 is a graph showing change in inducer blade pitch along the axiallength of the inducer; and

FIG. 10 is an elevational view, partially in section, of two axiallyspaced pumps for separate or combined operation in accord with thepresent invention.

While the invention will be described in connection with the presentlypreferred embodiments, it will be understood that it is not intended tolimit the invention to these embodiments. On the contrary, it isintended to cover all alternatives, modifications, and equivalents asmay be included in the spirit of the invention.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention describes a rotary blood pump which has animproved rotor control system. The rotary pump has a pump blade geometryoptimized by a method of the present invention to provide high pumpefficiency while minimizing hemolysis and thrombus (hemolysis is definedquantitatively hereinafter). The pump requires less than 10 watts ofpower to pump 5 liters/minute against a pressure head of 100 mm Hg. Apreferred embodiment of the pump weighs 53 grams, has a length of 75 mm,and a diameter of 25 mm. An index of hemolysis of 0.018 g/100 literspumped has been achieved, although using the method of this invention,further reductions are possible. For reference, a more standard rollerpump has an index of hemolysis of 0.06 g/100 liters. Published articlesconcerning aspects of the present invention are hereby incorporated byreference and include the following: (1) "In Vitro Performance of theBaylor/NASA Axial Flow Pump", Artificial Organs, 1993 Volume 17, number7, page 609-613; (2) "Development of Baylor/NASA Axial Flow VAD,Artificial Organs, 1993, Volume 17, page 469.

Referring now to the drawings, and more particularly to FIG. 1, there isa shown a rotary blood pump 10 in accord with the present invention.Blood pump 10 includes a preferably metallic tubular pump housing 12which is, in a preferred embodiment, a straight-sided cylinder. Pumphousing 12 has a smooth inner bore wall 15 to minimize thrombusformation. Pump housing 12 defines a blood flow path 13 therethrough inthe direction indicated by blood flow arrows shown in FIG. 1.

Front and rear clamps 14 and 16, respectively, are used to secure flowstraightener 18 and diffuser 20 within pump housing 12. Pump housing 12is sufficiently thin-walled so that the tightening of clamps 14 and 16with clamp screws 22 locally deforms pump housing 12 about flowstraightener 18 and diffuser 20 to affix these components in position.The clamps provide a very convenient means of securing the rotorassembly. Alternatively, other means for securing these components couldbe used, such as spot welding, fasteners, and interference fit.

Flow straightener 18 serves two basic functions: (1) it straightensblood flow to reduce hemolysis while improving pump efficiency, and (2)it provides a support structure for front ball-socket bearing assembly24, as discussed hereinafter. By straightening the flow of blood as itinitially flows into the entrance 36 of pump 10, hydraulic efficiency isincreased. Straightening the blood flow also reduces turbulence toincrease the pump pressure. FIG. 7 lists optimal values and permissiblepump parameter ranges for the flow straightener 18 and the other pumpcomponents.

Flow straightener 18 preferably has four fixed blades 26, but could haveonly two blades. Too many blades impede blood flow, while too few bladesreduces pump efficiency. For purposes of lowering thrombosis, the frontedge 28 of each blade 26 is sloped from inner housing wall 15 to flowstraightener hub 32 so that blood trauma by contact with blades 26 isminimized. Also to reduce blood trauma, flow straightener hub 32 iscylindrical with a round leading surface 34. Surface 34 may also behyperbolical or generally bullet-shaped for this purpose. An alternativeembodiment flow straightener 18a is shown in FIG. 3, and does not havethe sloping front edge blades.

The preferred angle of attack of blades 26 is 90°, i.e., the bladeswould intersect a plane transverse to cylindrical housing 12 at an angleof 90°. This reference for the angle of attack or pitch of the bladeswill be used throughout this specification.

Flow straightener 18 is preferably metallic, but may also be formed ofplastic. If formed of plastic and secured in place by clamp 14, it isnecessary to reinforce flow straightener 18 with, for instance, metallicsupports to prevent plastic creep deformation. The plastic creepdeformation phenomena might otherwise eventually cause flow straightener18 to come loose from clamp 14. Reinforcement is also necessary withrespect to other clamped plastic pump components.

Diffuser 20 also has two basic purposes: (1) it de-accelerates andredirects the outflow at blood flow path exit 40 axially to boost pumpperformance, and (2) it serves as a support structure for the rear rotorbearing 42. Diffuser 20 preferably has from 5 to 8 fixed blades 38, with6 blades being presently preferred. Blades 38 are fixably engaged withpump housing 12 after rear clamp 16 is tightened by screw 22.

To perform the function of de-acceleration and axial redirection ofblood flow, each diffuser blade 38 has an entrance angle of from about10° to 25° for slowing the blood down, and an outer angle of from about80° to 90° for redirecting blood flow in an axial direction. Presentlypreferred blade geometry is listed in FIG. 7 and includes an entranceangle of 15° and an outlet angle of 90°. Tail cone 44 of diffuser 20 ishyperbolical or generally bullet-shaped to reduce turbulence or wake ofblood flow from pump 10 so as to minimize blood damage from suchturbulence. Somewhat surprisingly, it was found that increasing thenumber of fixed blades tends to decrease hemolysis.

Rotor 46 is supported for rotary movement with pump housing 12 by frontand rear bearings 24 and 42, respectively. Rotor 46 is divided into twoportions along its axis based on the type and function of the bladesdisposed thereon. Inducer portion 48 is disposed in the from part ofrotor 46, i.e., nearer to the pump inlet 36. Impeller portion 50 isdisposed in the rear part of rotor 46 closer to pump outer 40. It hasbeen found that including an inducer portion in an axial flow pump,along with an impeller portion, significantly reduces hemolysis.

FIG. 3 shows an alternative embodiment pump of the present inventionwhich provides for two distinct sets of axially spaced blades which moreclearly distinguish the inducer portion of the rotor from the impellerportion of the rotor. Corresponding components of pump 10 and 10a aregiven the same number, with the difference of an "a" suffix todistinguish the components for comparison purposes as necessary. Areference to one number is therefore a reference to its correspondingnumber in this specification, unless otherwise stated. Where componentsare substantially different between the two pump versions, completelynew numbers are assigned. In pump 10a, inducer portion 48a is separatedfrom impeller portion 50a of rotor 46a by gap 49a, which is preferablyless than 0.10 inches. Inducer blade 52a may be tapered (not shown) atforward end 56a so that blade 52a has a smaller radial length at forwardend 56a, perhaps even blending into hub 73. However, using the method ofthe present invention, it has been found that a continuous blade pumphas even more reduced levels of hemolysis than the non-continuous bladepump 10a. Thus, pump 10 shown in FIG. 1 is the presently preferredembodiment.

Inducer blades 52 on inducer 53 have a variable pitch along their axiallength. FIG. 9 shows an inducer blade angle profile that plots angle ofattack in degrees versus axial position on inducer 53 in inches. It wasfound that the inducer portion 48 reduced hemolysis by approximately 45%from a pump design without the inducer. Hydraulic efficiency was alsoincreased as the rotation speed required to pump 5 liters/min of bloodat 100 mm Hg. dropped from 12,600 rpm to 10,800 rpm. The inducer blades52 pre-rotate the blood before it enters the main pumping or impeller 54to reduce hemolysis.

Inducer blades 52 also achieve a pumping action that effectivelyproduces a two-stage, increased efficiency pump. Optimum inducer bladegeometry for minimal hemolysis and maximum pump efficiency is listed forspecific parts of inducer blade 52 in the chart of FIG. 7. Thus, theentrance angle of leading end 56 of inducer blade 52 is preferably 10°,but has a preferred range from about 10° to 20°. The shallow entranceangle effectively engages the blood for movement without damaging theblood. The pitch of inducer blade 52 continues to change along its axiallength, and preferably is about 30° at midpoint 58 of the blade. Thetailing end 60 of inducer blade 52 preferably has an outlet angle of20°. This variable pitch is described in FIG. 9, which shows how pitchvaries with the axial length of inducer 53. As well, it is desired thatinducer blades have a wrap of preferably 240° around rotor 46. Thepresently preferred overlap of each blade over other blades is 120° or a50% overlap. The chart of FIG. 7 provides a complete listing of relevantpump parameter values, including preferred ranges of operation. Outsideof these ranges, pump efficiency drops and/or blood damage is morelikely to occur.

Pump 10 includes an interconnecting blade portion 62 which is notincluded in pump 10a. Although the two-stage pump 10a producessignificantly reduced hemolysis and efficient pump operation compared toa single stage pump, it has been discovered that by interconnectinginducer blades 52 with impeller blades 54 with interconnecting bladeportion 62, hemolysis may be reduced to even lower levels whilemaintaining efficient pump operation.

Impeller blades 54 on impeller 55 have an entrance angle in leading endregion 64 of preferably 20°. This may be seen more clearly in FIG. 3,which has no interconnecting blade portion 62. The entrance anglepreferably smoothly tapers to an optimum preferred outlet angle of 90°at blade tailing end region 66. The optimum ranges of operation for theentrance and outlet angles is given in FIG. 7.

Impeller blades 54 include axially longer impeller blades, such aslonger blade 68, and axially shorter impeller blades, such as shorterblade 70. The alternate long and short blade arrangement on impeller 55accommodates multiple magnetic poles for electric motor operation asdiscussed hereinafter, and still maintains adequate flow area throughimpeller 55. Presently, the preferred number of impeller blades is six,but a range from two to six blades provides permissible pump efficiency.If impeller 55 included six axially longer blades, such as longer blade68, the flow area through impeller portion 50 is restricted to such anextent that the blades actually begin to block the flow they areintended to produce.

Using the method of this invention, it was unexpectedly discovered thathemolysis does not necessarily increase with the number of blades, asanticipated. The alternating long-short blade arrangement of the sixbladed impeller of the present invention does not cause hemolysis anymore significantly than a two-bladed impeller. In some cases, animpeller with four long blades may cause more hemolysis than either atwo or six bladed impeller. It is possible that the degree of hemolysisdepends more on the number of long blades rather than the total numberof blades.

FIG. 4 shows a portion of impeller 55 in cross-section to illustrate thesubstantially flat, non-radiused blade tips 72. It has been unexpectedlyfound, using the method of this invention, that flat or substantiallyflat, non-radiused blade tips have substantially the same pump response,but do not produce significantly different hemolysis results fromrounded or radiused blade tips. It was anticipated that flat blade tipswould produce higher hemolysis. Because flat blade tips are lessexpensive to manufacture in conjunction with magnets to be used in theblades as discussed hereinafter, flat blade tips comprise the presentlypreferred embodiment. Further test results are discussed in greaterdetail in the previously noted articles incorporated herein byreference.

FIG. 4 also illustrates a preferred rotor hub 73 with outside diameter74 compared to the overall outside diameter 76 of inducer 53 and/orimpeller 55. The preferred ratio is 0.48, although a range of 0.45 to0.55 permits excellent pump operation. If the hub is smaller thanpermitted by this range, blood becomes excessively swirled and may tendto recirculate within pump 10 in the wrong flow direction to possiblydamage the blood as well as reduce pump efficiency. If the hub is toolarge so as to be outside of this range, the hub tends to block flowthrough the pump 10.

The radial clearance 78 between inducer 53 and/or impeller 55 withrespect to the pump housing inner wall 15 is preferably in the range ofabout 0.003 inches to 0.015 inches. Using a test matrix as per themethod of this invention, it was unexpectedly discovered that smallerradial clearances lowered hemolysis. It was expected that smallerclearances would produce greater blood damage due to higher sheerstresses on the blood. The presently preferred radial clearance 78 isabout 0.005 inches. Axial clearances between components, such as flowstraightener 18, rotor 46, and diffuser 20, are shown in FIG. 7. Axialclearances should be within the ranges shown to improve pump efficiencyand to reduce hemolysis.

In order to reduce the air gap between stator 80 and magnets 82, themagnets are preferably sealingly mounted within impeller blades 54.Reducing the air gap between the stator 80 and the magnets 82 increasesmotor efficiency, because magnetic flux is not as diffused as in motordesigns with large air gaps. The preferred radial spacing or air gapbetween magnets 82 and stator 80 is from 0.01 inches to 0.025 inches.Magnets 82 are preferably rare earth magnets because of the highermagnetic flux produced by such magnets. Each magnet 82 is encapsulatedin an individual pocket 84 to eliminate corrosion. Because magnets areindividualized, motor torque and rotor weight can be easily adjusted inthe manufacturing stage to provide motors that are tailored to the typeof pump performance necessary without producing excessive pump weight.

Field winding 88 generates a magnetic field to rotate rotor 46. Stator80 is comprised of individual stator laminations 86 to eliminate eddycurrents that generate heat and reduce efficiency. Heat flow from pump10 is directed both into the blood stream and into the tissuessurrounding pump 10. In this way, the blood is not heated in a mannerthat may damage the blood and, as well, the surrounding tissues do nothave to absorb all the heat. Heat conductive flow paths using thermallyconductive material, such as the metal of the stator or a thermallyconductive gel, may be used to provide approximately the desired ratioof heat flow to the tissues compared to the heat flow to the blood.

As illustrated in FIG. 2, stator 80, comprised of individual laminations86, is stacked in a presently preferred skewed manner such that pathways90 provide a motor winding pathway that is offset from the rotor axis92. The skew of laminations 86 may or may not correspond in some mannerwith the offset angle or changing offset angle of the row of magnets 82,and is not limited to the position shown in FIG. 2. A skewed stator 80is also indicated in FIG. 2A, which shows an offset path from axis 92for the field windings 88 which travel through stator 80. The skewingangle or offset from the rotor axis is used to optimize performance. Theskewing angle of stator 80 may be variable rather than fixed along itslength. While skewed stator 80 is the presently preferred embodiment,other factors or combinations of factors (e.g. small air gap, magnetorientation, etc.) produce excellent pump and motor performance withouta skewed stator.

An axial force is produced on rotor 46 during rotation, which can bevaried by moving stator 80 axially along pump housing 12. Stator 80 isaxially adjustable for this purpose, and could be fixed in positionduring manufacturing for optimal performance given the number of magnetsto be used and given other factors discussed hereinafter. The axialforce thus produced can be used to offset the thrust created duringpumping to reduce the load on the front or rear bearing assemblies 24and 42, respectively. The axial positioning of stator 80 may also beused to optimize electrical motor efficiency.

Referring to FIG. 5, a block diagram of the control system 100 of thepresent invention is shown. Note that control system 100 may operate twomotors 1 and 2, such as shown in FIG. 10. For some applications, eitherfor implantation or for external use, it may be desirable to have twopumps connected either in parallel or in series. Thus, control system100 can be easily configured for this purpose if desired. In addition,magnets (not shown) may be placed in the inducer hub to provide asecondary motor in the case of primary motor or controller failure.Various other back-up and redundancy configurations may be used.

For instance, in an axially spaced pump configuration shown in FIG. 10,motors 1 and 2 are axially displaced from each other and may be operatedseparately or in conjunction with each other as control system 100regulates power, as discussed hereinafter, to axially spaced stators 207and 208 containing stator windings 210 and 212, respectively.Ball-socket bearing 203 on modified diffuser 204 rotatably supportsrotor 205 of motor 2, which is axially spaced from rotor 206 of motor 1.Diffuser 204 acts as a flow straightener when independent operation ofmotor 2 is desired. Control system 100 may be used to operate bothmotors simultaneously or to turn one motor on if micro-controller 102senses that another motor has failed. Micro-controller 102 may beprogrammed for pulsatile motor operation or continuous speed motoroperation of one or more motors, as desired.

If only one pump is to be used, extra components may be removed. In FIG.5, except for micro-controller 102, most components are duplicated toallow for operation of the two motors. For convenience, reference tocorresponding components will be made to one number, with thecorresponding component having an "a" suffix. Control system 100operates either manually or by micro-control as discussed subsequently,and may be used for test purposes if desired.

Control system 100 applies current to stator windings 88. Preferablystator 80 includes three stator windings 88. Stator 80 generates arotating magnetic field which magnets 82 and thus rotor 46 follow toproduce motion. The motor stator may be three phase "Y" or "Delta"wound. The operation of the brushless D.C. motor of the presentinvention requires a proper sequence of power to be applied to statorwindings 88. Two stator windings 88 have power applied to them at anyone time. By sequencing the voltage on and off to the respective statorwindings 88, a rotating magnetic field is produced.

A preferred embodiment three-phase motor requires a repetitive sequenceof six states to generate a rotating magnetic field, although othercommutation schemes could be used. The six states are produced byelectronic commutation provided by power F.E.T.'s 104. If Motor 1 weresequenced through the six electrical states at a controlled frequencywithout feedback, its operation would be that of a stepper motor. Insuch a mode of operation, the motor rapidly loses its ability togenerate torque as the rpm's increase.

Control system 100 detects back electromotive force or back emf tocontrol motor operation. Whenever a conductor, such as field winding 88,is cut by moving magnetic lines of force, such as are generated bymagnets 82, a voltage is induced. The voltage will increase with rotorspeed. It is possible to sense this voltage in one of the three statorwindings 88 because only two of the motor's windings are activated atany one time, to determine the rotor 46 position, and to activatecommutator switches 104. The circuitry is much simpler and more reliablethan Hall Effect sensors which have been used in the past. Although aback emf control is the presently preferred embodiment, a Hall effectdriven commutation scheme could also be used.

Back emf integrated circuit 106 provides sensors for detecting the backemf from lines indicated at 107, and operates commutation switches 104accordingly. A presently preferred back emf integrated circuit includesa ML4411 motor controller integrated circuit. Each commutation switchF.E.T. is preferably turned all the way on or off for maximum powerefficiency.

Back emf integrated circuit 106 also provides a start up mode operationwhen the back emf is not present or large enough for detection. Fromzero to approximately 200 rpm's, motor 1 operates in stepper motorfashion as described hereinbefore. Motor speed is controlled with adifference amplifier 108, which may take its speed signal from eithermicro-controller 102 or speed adjust pot 110 as selected by switch 112.A speed detection signal is available from the back emf integratedcircuit 106 for this purpose.

Restart circuit 110 and micro-controller 102 monitor voltage developedacross sense resistor 111 (present preference is about 0.1 ohms) and thefrequency signal from back emf integrated circuit 106 to determinewhether motor 1 should be restarted, i.e., due to a sudden increase ordecrease in current or frequency. Switch 113 may be used to selectbetween use of restart circuit 110, micro-controller 102, or a manualrestart switch 114. Controller 102 may be programmed to produce an alarmsignal if there are sudden changes in power consumption or frequency, asmay occur if heart strength weakens or improves. To protect theelectronics from electromagnetic interference (EMI), ferrite beads 116are used with wires to an external power supply. The electronics arepreferably hermetically sealed in case 118, which is formed of a high mumaterial to limit EMI.

Referring now to preferred bearing configuration embodiments, FIG. 6Adiscloses a presently preferred ball-socket bearing configuration. Thebearings are comprised of bio-compatible material and, in the presentlypreferred embodiment, are comprised of ceramic material. The ball-socketbearing 121 may be configured as shown in FIG. 1, or may be otherwiseconfigured. Ball 120 is preferably secured by some securing means, suchas glue or welding, along edge 128. Ball 120 could be molded into onecomponent, or split and secured as is known in the art. Ball 120 has aspherical surface 122 that engages a mating seat spherical surface 124.A void or bearing chamber 126 is filled with a bio-compatible materialto prevent blood from coming into this area and stagnating. In apreferred embodiment, bearing chamber 126 is left empty and allowed tofill with blood. The blood cross-links due to bearing heat and takes ona soft, pliable, plastic texture. The cross-linked blood may perform, tosome extent, a bearing surface function. The cross-linked blood thenprevents other blood from entering the bearing and stagnating.

While the ball-socket bearing is the preferred configuration, anotherconfigurations may be used. Shaft-journal bearing 130 provides shaft 132secured to component 134, which may be rotor 46, for rotation with asecond component 136, which may be diffuser 20. Journal sleeve 138 has acylindrical bearing receiving surface 140 for engaging the cylindricalshaft bearing surface 142. A bio-compatible material in bearing chamber144 is preferably cross-linked blood, as discussed hereinbefore, whichhas leaked into this chamber, been heated, and solidified. Anotherpreferred embodiment of a shaft-journal bearing would include a shaft(not shown) extending through rotor 46 and engaging respective sleeveson flow straightener 18 and diffuser 20. As well, the flow straightenerand/or diffuser, or relevant portions thereof, may be made from asuitable material, such as zirconia, with the bearing surfaces beingformed or machined directly into that material. In a similar manner, thebearing surfaces of bearing 121, 130, or other bearings could bemachined into a bearing mount component.

To avoid the problem of blood stagnation in the region of the bearing,several bearing washing configurations, using directed blood flow orpump differential pressures, may also be used as part of the presentinvention. Referring to FIG. 3, there is shown a means for using a pumppressure differential for washing bearing 150 which includes shaft 152and journal sleeve 153. High pressure region 154 produces a blood flowthrough passage 156 to wash bearing 150 and exit to lower pressureregion 158. Thus, blood is prevented from stagnating behind bearing 150.

Another bearing embodiment (not shown) would include a pressure fedjournal bearing to form a hydrodynamic film that supports the shaft andloads so that the bearing surfaces do not touch. Shaft 152 would beslightly undersize with respect to a journal sleeve for this purpose. Apressure fed flow passage may be directed through such a hydrodynamicjournal bearing from a high pressure region 154 to a low pressure region158.

FIG. 6C illustrates another bearing washing method which includes aproducing bend 160 in pump housing 12. This bend places bearing 162 in ahigh velocity flow area. Blood flow through bend 160 washes the bearingmembers, which include a male portion 164 and mating member 166. Otherconfigurations for this type arrangement may be used, but the generalprinciple is as described. The flow straightener is removed in thisembodiment, but may be included in another embodiment where it does notnecessarily need to act as a bearing support.

FIGS. 6D and 6E illustrate yet another bearing arrangement embodiment.In this embodiment, a bearing shaft 170 extends through journal sleeve172. Bearing shaft 170 has an oblong or substantially oval transversecross-section, as shown in FIG. 6E. This configuration produces flowpaths 174 through journal sleeve 172 so as to flush the bearing andprevent blood stagnation. Shaft 170 could also be fluted so as to havespirals, slots, or other flow paths 174 formed along its peripherywithin sleeve 172.

FIG. 6F shows a bearing configuration wherein rotor 46 is cantileveredwith respect to diffuser 20 using two bearings 180 and 182. Shaft 184extends between the two bearings. Each bearing includes a rotating ballsurface, 186 and 188, mating with a socket surface 190 and 192,respectively. Socket surfaces 190 and 192 are formed in ceramic materialsleeves 194 and 196, respectively. The bearings are "zero tolerance"bearings. Thus, either the shaft 184 which runs through diffuser 20 orthe diffuser 20 itself must provide a takeup mechanism that keeps eachball surface 186 and 188 tightly engaged with a respective socketsurfaces 190 and 192. For this purpose, diffuser 20 could be made ofcompressible material. Alternatively, diffuser 20 could receiveinjection molding around sleeves 194 or 196.

As another alternative, either a passive, active, or a combinationpassive-active magnetic bearing suspension system (not shown) could beused to rotatably mount rotor 46. For this embodiment, the rotor 46would be axially positioned and/or bearing surfaces would be suspendedwith respect to each other using magnetic force.

The method of the present invention is illustrated in FIG. 8 which showsa test matrix for optimizing the pump parameters which are believed toaffect hemolysis. This method enables optimization of pump parameters asdiscussed hereinafter with a minimum number of tests. Although thismatrix is designed for a blood pump with no inducer or flowstraightener, it is believed that the method of the present invention isclearly illustrated with this example, and may be used to improve mostother blood pump designs with respect to hemolysis, thrombus, pumpefficiency, cost, and other important factors. Using this approach, itis possible to thoroughly investigate many parameters in an organized,methodical approach to achieve highly desired goals, such as optimumpump performance and minimum hemolysis.

To apply this method, it was first necessary to identify the pumpparameters that were believed to affect hemolysis. The variables arelisted at the headings of the test matrix, and include blade tip shape,radial clearance, axial clearance, number of blades, and impellerlength. To judge effectively the impact of each variable on hemolysis,the method of the present invention requires that all pump dimensions beheld constant while only one variable is changed. As shown in the matrixof FIG. 8, 16 different tests are used. Preferably, each test is made atleast three different times to provide statistical results and checkconsistency.

The information learned in initial tests is used later in the matrix.For example, tests 1 and 2 compare the effects of flat and round bladetip geometries. The least hemolytic of the two is then used in allremaining tests. If there is no statistically significant difference,then the result that provides superior hydrodynamic performance is used.If there is no hydrodynamic difference, then the least expensiveparameter to manufacture is used. Test conditions are constant for eachparameter. All tests were made using bovine blood at a flow of 5liters/min against 100 mm Hg. The duration of each test was 20 minutes.The pump circulates blood through a test loop having a 500 cc bloodreservoir which was conveniently a 500 cc blood bag. The 500 cc bloodbag is preferably changed after each version of each parameter istested.

In order to compare various impeller and stator blade geometries withina reasonable time period, a stereolithography technique was used toquickly realize the complex shapes. This technique relies on a laserbeam that scans the surface of a liquid acrylic polymer. The polymerhardens under the influence of the laser, and layer by layer a solidshaped is formed from the liquid surface.

To compare hemolysis results, an index of hemolysis (IH) is used. Thisis defined as the amount of hemoglobin liberated in grams per 100 litersof blood pumped against 100 mm Hg. In equation form: ##EQU1##

IH equals amount of hemoglobin liberated in grams per 100 liters ofblood pumped against 100 mm Hg;

Ht is the hematocrit in decimal percent;

V is the blood volume in liters;

ΔHb is the amount of hemoglobin liberated in a fixed time period ingrams/liter;

Flow is the flow rate in liters per minute; and

Time is the total time in minutes at that flow rate.

The final results are seen in the matrix of FIG. 8. Other more detailedfactors of testing in accord with the method of this invention arediscussed more thoroughly in the articles which have been namedpreviously and incorporated herein by reference.

As to manufacturing and usage considerations, pump 10 is preferablymanufactured using materials designed to be buoyant inside the body tomake the completed pump neutrally buoyant or approximately neutrallybuoyant. This minimizes stress on stitches or other means used toposition the pump within the body. Thus, the rotor, rotor blades, and/orother components may be made with a lightweight material havingsufficient thickness to produce a buoyant effect.

Pump 10 has numerous uses as a blood pump, including use as a portableblood pumping unit for field service. Pump 10 may also be used for otherclinical applications involving other fluids. It could, for instance, beused in a compact heart-lung machine. Due to the small volume and sizeof pump 10, it can be placed close to a patient to minimize shock causedwhen initiating blood pump operation using a saline solution. Largerpumps, with larger volume, may be awkward to move close by a patient toeliminate this shock.

Thus, the blood pump of the present invention, particularly whenoptimized using the method of the present invention, has many advantagesover the prior art. For instance, there are no blood seals which requirebearing purge systems. No Hall Effect sensors are required which maytend to limit motor control reliability due to their complexity. Aswell, pump 10 provides low power consumption and very low levels ofhemolysis.

The foregoing disclosure and description of the invention isillustrative and explanatory thereof. It will be appreciated by thoseskilled in the art that various changes in the size, shape, materials,as well as in the details of the illustrated construction, may be madewithout departing from the spirit of the invention.

What is claimed is:
 1. A fluid pump, comprising:a pump housing having afluid flow path therethrough; a first stator mounted to said pumphousing, said first stator having a first stator field winding forproducing a first stator magnetic field; a first rotor mounted withinsaid pump housing for rotation about a longitudinal axis of said firstrotor in response to said first stator magnetic field, said first rotorcarrying a blade thereon for propelling fluid through said pump housingalong said fluid flow path, at least a portion of said blade beingaffixed to said rotor at an angle that is offset with respect to saidlongitudinal axis; a plurality of magnets secured to said at least aportion of said blade affixed to said rotor at an angle that is offsetwith respect to said longitudinal axis, said plurality of magnetsproducing a first rotor magnetic field that passes through said firststator field winding during said rotation of said first rotor to inducea back emf within said first stator field winding; and back emf sensorcircuitry connected to said first stator field winding to sense saidback emf produced during said rotation of said first rotor within saidpump housing.
 2. A fluid pump, comprising:a pump housing definingtherein a fluid path for fluid flow through said pump; a rotor mountedwithin said pump housing for rotation therein about a rotor axis;impeller means for moving a fluid through said fluid path, said impellermeans comprising at least one impeller blade on said rotor; a pluralityof magnets secured to said rotor; a stator in surrounding relationshipto said pump housing, said stator and said plurality of magnets beingpositioned to define a magnetic flux gap therebetween having a radialspacing of less than 0.025 inches.
 3. The pump of claim 2, furthercomprising:at least one rotor mount for rotably securing said rotor; andat least one clamp for securing said at least one rotor mount withinsaid pump housing, said pump housing being sufficiently thin-walled suchthat tightening of said at least one clamp acts to deform said pumphousing so as to affix said at least one rotor mount thereto.
 4. Thepump of claim 2, wherein:said plurality of magnets are arranged inhelical fashion about said rotor, said plurality of magnets beingmovable with respect to said stator.
 5. The pump of claim 2,wherein:said stator is adjustable with respect to said pump housing andsaid rotor along an axis substantially collinear with said rotor axis.6. The pump of claim 2, further comprising:a radially peripheral edge ofsaid at least one impeller blade, said plurality of magnets beingmounted along said radially peripheral edge, at least two of saidplurality of magnets being in close proximity.
 7. A rotary motor,comprising:a motor housing; a rotor mounted within said motor housingfor rotation about a rotor axis, said rotor having a circumference; astator mounted to said motor housing to produce a magnetic field forrotatably driving said rotor; and a plurality of individual permanentmagnets positioned along said circumference of said rotor with avariable axial position with respect to said rotor axis such that saidplurality of individual permanent magnets define at least a portion of ahelical pattern and are moveable with respect to said stator.
 8. Therotary motor of claim 7, further comprising:at least one clamp disposedaround said motor housing, said motor housing being sufficientlythin-walled to facilitate deformation of said motor housing for mountingof said rotor by tightening of said at least one clamp.
 9. The rotarymotor of claim 7, wherein:said plurality of individual permanent magnetsare mounted with a radial magnetic flux gap spacing to said stator ofless than 0.025 inches.
 10. The rotary motor of claim 7, furthercomprising:at least one impeller blade mounted to said rotor, saidplurality of individual magnets being affixed along a peripheral edge ofsaid at least one impeller blade such that at least two of saidindividual permanent magnets are closely proximate to each other alongsaid peripheral edge.
 11. The rotary motor of claim 7, wherein saidstator further comprises:a plurality of laminations, said laminationsbeing stacked together such that a passageway is formed therein, saidpassageway being skewed with respect to said rotor axis.
 12. The rotarymotor of claim 7, wherein:said at least one impeller blade has avariable pitch.
 13. A rotary motor, comprising:a motor housing; a rotormounted within said motor housing for rotation about a rotor axis, saidrotor having a circumference; a plurality of individual permanentmagnets positioned along said circumference of said rotor; and a statormounted to said motor housing to produce a magnetic field for rotatablydriving said rotor, said stator comprising a plurality of laminations,said laminations being stacked together such that a passageway formedtherein is skewed with respect to said rotor axis, a stator windingwithin said passageway that follows said passageway so as to be skewedwith respect to said rotor axis.
 14. The rotary motor of claim 13,wherein:said stator is axially adjustable with respect to said motorhousing.
 15. The rotary motor of claim 13, further comprising:a clampexternal to said motor housing for securing said rotor axially withrespect to said motor housing.
 16. The rotary motor of claim 15,wherein:said motor housing is sufficiently thin-walled to be deformableby said clamp.
 17. The rotary motor of claim 13, wherein:said pluralityof individual permanent magnets include more than two individualmagnets.
 18. A fluid pump, comprising:a pump housing defining therein afluid path for fluid flow through said pump; a rotor mounted within saidpump housing for rotation therewith; a rotor mount within said pumphousing for rotatable mounting of said rotor within said pump housing;and a clamp surrounding an external portion of said pump housing, saidpump housing being deformable in the locus of said clamp for securingsaid rotor mount within said housing by tightening of said clamp. 19.The fluid pump of claim 18, wherein:said rotor mount is comprised ofreinforced plastic to prevent plastic creep deformation.
 20. The fluidpump of claim 18, further comprising:a plurality of magnets secured tosaid rotor.
 21. The fluid pump of claim 20, further comprising:a statorsurrounding said pump housing, said plurality of magnets beingpositioned radially outwardly on said rotor to minimize a gap formedbetween said plurality of magnets and said stator.